1. Field of the Invention
The present invention relates to an optical space transmission device performing space transmission of signals using light. More specifically, the present invention relates to an optical space transmission device suitable for use in an application performing wireless signal transmission within a device which includes a rotation mechanism such as a monitoring camera.
2. Description of the Related Art
In a conventional device including a rotation mechanism, for example such as a monitoring camera, generally, a fixed portion and a rotating portion are electrically connected by a slip ring, through which signals such as control and video signals are transmitted between the fixed and rotating portions as disclosed in Patent Literature 1 (Japanese Patent Laid-open Publication No. 2000-83181), for example. However, such signal transmission using the slip ring is reaching the limit of performance because various types of control signals are coming into use along with sophistication of devices and volume of data is increasing along with an improvement in image quality.
On the other hand, in recent years, techniques of space transmission of signals using light, such as short-range wireless communication using infrared rays, for example, IrDA, are dramatically advancing and are widespread as the technique to implement wireless signal transmission between devices spatially apart from each other (for example, see Patent Literature 2 (Japanese Patent Laid-open Publication No. 11-41301). Such a technique of space transmission of signals using light is thought to be effectively applied to signal transmission between the fixed and rotating portions of the device including a rotation mechanism such as a monitoring camera.
Meanwhile, the short-range infrared wireless communication such as IrDA uses a light emitting diode (LED) as a light source emitting light modulated based on transmitted signals. The limit of modulation speed of this LED often determines the upper limit of the transmission speed. Accordingly, when the aforementioned technique is directly applied to the signal transmission between the fixed and rotating portions of the monitoring camera, for example, it may be possible to achieve transmission speed high enough to transmit a control signal from the fixed portion including a controller to the rotating portion including a camera section. However, in this case, it may be not possible to achieve transmission speed high enough to transmit a video signal including data of large size from the rotating portion including the camera section to the fixed portion including the controller, and it is thought to be difficult to properly transmit the video signal.
Patent Literature 2 proposes a technique which substantially enables high speed transmission using an LED as a light source by providing a circuit which performs signal processing so that the LED emits multiple-valued signal light which varies stepwise. However, such a method requires a dedicated signal processing circuit, therefore causing an increase in costs. Moreover, it is difficult to transmit the video signal at real time.
Another example of space transmission of signals using light which is utilized in an application transmitting a video or audio signal from a video reproducer to a television without wires uses a semiconductor laser (LD) as a light source. LDs have maximum modulation speed dramatically higher than LEDs and therefore can be adequately applied to transmission of a video signal including data of large size.
However, some parts including an LD and an LD driver driving the LD for modulation of light are more expensive than LEDs and an LED driver. Accordingly, employment of this method for two-way signal transmission between the fixed and rotating portions in the monitoring camera is not efficient and increases costs. Such a problem can be caused not only in the monitoring camera but also in every device which requires two-way signal transmission between the fixed and rotating portions through the rotation mechanism and requires high-speed transmission only in one way.